Systems and methods for providing ac power from multiple turbine engine spools

ABSTRACT

Systems and methods for providing AC power from multiple turbine engine spools are disclosed. An aircraft system in a particular embodiment includes an engine having a first shaft connected and a second shaft. The aircraft system can further include a bus system and a first energy converter including a starter/generator, coupled between the first shaft and the bus system to convert a first variable frequency energy transmitted by the first shaft to a first generally constant frequency energy. A second energy converter can be coupled between the second shaft and the bus system, with the second energy converter including a generator to convert a second variable frequency energy transmitted by the second shaft to a second generally constant frequency energy.

TECHNICAL FIELD

The present disclosure is directed generally to systems and methods forproviding alternating current (AC) power from multiple turbine enginespools, for example, constant frequency AC power from multiple spools ofan aircraft turbofan engine.

BACKGROUND

Modern commercial transport aircraft are typically driven by two or morehigh bypass ratio turbofan engines. These engines include a fan thatprovides a significant fraction of the overall propulsion system thrust.An engine core drives the fan as well as one or more compressors, andproduces additional thrust by directing exhaust products in an aftdirection.

In addition to providing thrust to propel the aircraft, and powering theaircraft hydraulic and pneumatic systems, the turbofan engines provideelectrical power to many aircraft components, including theenvironmental control system, aircraft computers, hydraulic motor pumps,and/or other motors and electrical devices. One approach to obtainingelectrical power from the aircraft engines is to convert the rotationalmotion of the turbomachinery components to electrical power. While thisapproach has been generally effective, the manner in which the power isextracted from the engines is not always efficient. This in turn cancreate additional inefficiencies as automated aircraft systems and/orcrew compensate or overcompensate for an initially inefficient powerextraction. Accordingly, there remains a need for more efficienttechniques for extracting electrical power from aircraft turbofanengines.

SUMMARY

The following summary is provided for the benefit of the reader only,and is not intended to limit in any way the invention as set forth bythe claims. An aircraft system in accordance with a particularembodiment includes a turbofan engine that in turn includes acompressor, a first turbine, and a first shaft connected between thecompressor and the first turbine. The engine further includes a fan, asecond turbine, and a second shaft connected between the fan and thesecond turbine. The system can further include a power bus, a firstenergy converter coupled between the first shaft and the power bus, anda second energy converter coupled between the second shaft and the powerbus. The first energy converter can include a synchronousstarter/generator and can be positioned to convert a first variablefrequency energy transmitted by the first shaft to a first generallyconstant frequency energy. The second energy converter can include asynchronous generator and can be positioned to convert a second variablefrequency energy transmitted by the second shaft to a second generallyconstant frequency energy, with the second generally constant frequencyenergy in phase with and at generally the same frequency as the firstgenerally constant frequency energy. A controller can be operativelycoupled to the starter/generator and the generator, for example, tocontrol functions of these components.

In a further particular embodiment, the first energy converter caninclude a mechanical continuously variable transmission connectedbetween the first shaft and the starter/generator. The continuouslyvariable transmission can include a variable rotation rate input shaftand a constant rotation rate output shaft. In another embodiment, thestarter/generator can include a variable frequency generator and thefirst energy converter can further include an electrical invertercoupled to the starter/generator to receive a variable frequencyelectrical power and produce a constant frequency output power.

A method for operating an aircraft system in accordance with aparticular embodiment includes starting a turbofan engine by driving afirst shaft with a starter/generator, with the first shaft beingconnected between a compressor and a first turbine of the engine. Themethod can further include extracting a first portion of energy from thefirst shaft with the starter/generator, and extracting a second portionof energy from a second shaft connected between a fan and a secondturbine of the engine. The first portion of energy can be converted froma first variable frequency to a first fixed frequency, and the secondportion of energy can be converted from a second variable frequency to asecond fixed frequency generally identical to the first fixed frequency.The method can still further include distributing the first and secondportions of energy to aircraft components via a common electrical bus.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially schematic, isometric illustration of an aircraftthat can include power systems in accordance with embodiments of thedisclosure.

FIG. 2 is a schematic diagram illustrating components of a system forproducing AC electrical power from an aircraft engine in accordance witha particular embodiment.

FIGS. 3A-3D illustrate constant speed drives configured in accordancewith embodiments of the disclosure.

FIG. 4 illustrates an arrangement for providing constant frequency ACpower in accordance with another embodiment of the disclosure.

DETAILED DESCRIPTION

The following disclosure describes systems and methods for providingalternating current (AC) power, e.g., constant frequency power, frommultiple turbine engine spools, and associated systems, arrangements andmethods. Certain specific details are set forth in the followingdescription and in FIGS. 1-4 to provide a thorough understanding ofvarious embodiments of the disclosure. Other details describingwell-known structures and systems often associated with aircraft powersystems are not set forth in the following description to avoidunnecessarily obscuring the description of the various disclosedembodiments.

FIG. 1 is a partially schematic, isometric illustration of an aircraft100 that includes a fuselage 102, wings 101, and an empennage 103. Theempennage 103 can include horizontal stabilizers 104 and a verticalstabilizer 105. The aircraft 100 further includes a propulsion system110. In a particular embodiment shown in FIG. 1, the propulsion system110 includes two wing-mounted nacelles 112, each carrying a turbofanengine 111. In other embodiments, the propulsion system 110 can includeother arrangements, for example, engines carried by other portions ofthe aircraft 100 including the fuselage 102 and/or the empennage 103. Ina particular embodiment shown in FIG. 1, the engines 111 include highbypass ratio turbofan engines, but in other embodiments, the engines 111can have other configurations, including turbojet arrangements. In anyof the foregoing embodiments, mechanical energy is extracted from theengines 111 and converted to electrical energy to power a variety ofcomponents and systems on board the aircraft, including, but not limitedto environmental control systems, computer systems, electricalactuators, and electrical motors. Further details of arrangements forproviding the power in an efficient manner are described below withreference with to FIG. 2-4.

FIG. 2 is a schematic block diagram of a representative system 110 thatincludes one or more engines 111 (one is shown in FIG. 2 for purposes ofillustration) and associated components used to extract electrical powerfrom the engine 111. The engine 111 can include a compressor 113 housedwithin the nacelle 112 and coupled to a first turbine 116 a via a firstshaft 115 a. The compressor 113, first turbine 116 a and first shaft 115a can form a first, high pressure spool. The engine 111 can furtherinclude a fan 117 coupled to a second turbine 116 b via a second shaft115 b. The fan 117, second turbine 116 b and second shaft 115 b can forma second, low pressure spool. The second shaft 115 b can be positionedannularly inwardly from the first shaft 115 a so that the shafts canrotate at different speeds. In operation, the compressor 113 compressesincoming air, which is then provided to a combustor 114. Fuel isinjected into the compressed air and ignited at the combustor 114 andthe hot exhaust products are expanded through the first turbine 116 a todrive the compressor 113. The exhaust gases are further expanded throughthe second turbine 116 b to drive the fan 117, which directs bypass airaround the compressor 113, the first turbine 116 a and the secondturbine 116 b.

The system 110 can further include a first energy converter 130 aoperatively coupled to the first shaft 115 a and a second energyconverter 130 b operatively coupled to the second shaft 115 b. Thecouplings between the shafts 115 a, 115 b and the corresponding energyconverters 130 a, 130 b can include gear boxes or other devices thatextract rotational energy from the shafts 115 a, 115 b. The first energyconverter 130 a can include a first constant speed drive 150 a coupledto a synchronous starter/generator 132. The second energy converter 130b can include a second constant speed drive 150 b coupled to asynchronous generator 133. The synchronous starter/generator 132 canprovide both engine starting functions and electrical power generationfunctions, while the synchronous generator 133 typically provides onlyelectrical power generation functions.

The first and second constant speed drives 150 a, 150 b can beconfigured to receive a variable frequency input and provide a constantfrequency output. For example, as the speed of the first shaft 115 achanges during engine operation, the first constant speed drive 150 acan provide a constant speed output that is provided to the synchronousstarter/generator 132. Similarly, the second constant speed drive 150 bcan receive energy at a varying frequency from the second shaft 115 band can provide a constant speed output to the synchronous generator133. In addition, the speeds (e.g., rotational speeds or frequencies) ofthe outputs provided by the constant speed drives 150 a, 150 b can becontrolled, modulated and/or set to be identical or very similar.Accordingly, the power provided by the synchronous starter/generator 132will be at a first frequency and the power provided by the synchronousgenerator 133 will be at a second frequency that is identical or veryclose to the first frequency. In a further aspect of this embodiment,the electrical power provided by the starter/generator 132 is providedin phase with the electrical power provided by the generator 133.

The first energy converter 130 a and the second energy converter 130 bare both coupled to an electrical power bus 140. In a particularembodiment, the first energy converter 130 a is coupled to a firstportion 141 a of the bus 140 via a first contactor 142 a. The secondenergy converter 130 b is coupled to a second portion 141 b of the bus140 via a second contactor 142 b. The two portions 141 a, 141 b of thebus 140 are coupled via a tie switch or other coupling 143 that isnormally closed. The tie switch 143 can be normally closed because thepower provided by the first energy converter 130 a and the second energyconverter 130 b can be regulated to be at the same frequency. Inaddition, as noted above, the power provided by each of the two energyconverters can be regulated to be in appropriate phase anglerelationship with the other to achieve a desired output power sharingbetween the two converters. When the tie switch 143 is opened, thefrequencies can be very close to each other but need not be exactly thesame.

The first portion 141 a of the bus 140 can be coupled to a set of firstaircraft devices 144 a, and the second portion 141 b of the bus 140 canbe coupled to a set of second aircraft devices 144 b. During normaloperation, the first and second contactors 142 a, 142 b are closed, asis the tie switch 143. Accordingly, power is provided from the bus 140to both sets of aircraft devices 144 a, 144 b. In the unlikely eventthat the second energy converter 130 b or related particular systemcomponents fail, the second energy converter 130 b can be isolated fromthe bus 140 by opening the second contactor 142 b, while the firstenergy converter 130 a continues to provide power to both sets ofaircraft devices 144 a, 144 b. Similarly, if the first energy converter130 a or related components fail, the first energy converter 130 a canbe isolated from the bus 140 by opening the first contactor 142 a, whilethe second energy converter 130 b provides power to both sets ofaircraft devices 144 a, 144 b. Accordingly, the common bus 140 canprovide in-phase power to both sets of aircraft devices 144 a, 144 bwhen power is provided by both the energy converters 130 a, 130 b, andwhen power is provided by only one of the energy converters 130 a, 130b. In still a further mode, if either of the bus portions 141 a, 141 bwere to fail, the failed bus portion can be isolated by opening the tieswitch 143. In case of an engine failure that renders the energyconverters 130 a and 130 b inoperative, the bus 140 can be connected tothe corresponding bus(es) of one or more other engine(s) to power theaircraft devices 144 a and 144 b.

As discussed above, the AC power provided by each energy converter 130a, 130 b can match or approximately match the frequency of the AC powerprovided by the other, and can be in phase with the power provided bythe other. For example, in a particular embodiment, the energyconverters 130 a, 130 b can provide alternating current power at afrequency of about 400 Hz. The frequencies produced by the two energyconverters 130 a, 130 b can float relative to each other when the tieswitch 143 is open. When the tie switch 143 is closed, the energyconverters 130 a, 130 b can provide energy at identically the samefrequency. The phase angle difference between one converter relative tothe other can be adjusted to control the power sharing between theconverters. The constant speed drives 150 a, 150 b can provide thisconsistency despite large variations in the speeds with which the firstand second shafts 115 a, 115 b rotate. For example, the rotation rate ofthe first shaft 115 a can vary by a factor of about two between anengine idle condition and a full thrust condition. The rotation rate ofthe second shaft 115 b can vary by a factor of about five or morebetween engine idle and a full thrust. In a particular embodiment, therotation rate for the first shaft 115 a varies from about 5000 RPM toabout 10000 RPM, and the rotation rate for the second shaft 115 b variesfrom about 1000 RPM to about 5000 RPM. These ranges can have differentvalues for different engines, but generally, the range is greater forthe second shaft 115 b than for the first shaft 115 a. In any of theseembodiments, the power sharing arrangement between the two shafts canprovide engine benefits, for example, improving engine operability atlow power settings.

The ability to provide constant frequency, in-phase alternating currentpower to a common bus from two different shafts having widely varyingrotation rates can provide a variety of benefits. For example, oneexpected benefit of this arrangement is that the first shaft 115 a (e.g.the high pressure shaft) need not be relied upon exclusively forproviding electrical power to electrically driven aircraft devices. Overthe course of time, engine designers have increased aircraft enginebypass ratios in an effort to improve engine efficiency, and as aresult, a greater fraction of the total engine thrust is transmitted bythe second shaft 115 b and a lesser fraction by the first shaft 115 a.Accordingly, the power available for extraction from the first shaft 115a can be limited, particularly at low power settings. As a consequence,the operator (e.g., the aircraft power management computer or the pilot)may be forced to “shed” or shut down one or more of the aircraft devices144 a, 144 b during low engine power settings, to avoid extracting toomuch power from the first shaft 115 a. Alternatively, the operator canincrease the rotational speed of the engine 111 in order to provideenough power for all the desired electrical devices 144 a, 144 b.However, this may lead to an inefficient operation of the engine becausethe entire engine is driven at a higher rate simply to providesufficient electrical power. For example, if the engine power isincreased during flight, this can result in a thrust level that isgreater than necessary or desired, and can therefore increase fuelconsumption. If the engine power is increased on the ground beyond whatis required for the normal ground idle condition, the operator may needto ride the aircraft brakes to prevent aircraft overspeed, whichincreases the wear on the brakes. By extracting power from both thefirst shaft 115 a and the second shaft 115 b, both of the foregoingproblems can be avoided. In a particular embodiment approximately 50% ofthe power required by the aircraft devices 144 a, 144 b can be providedby the second shaft 115 b via the second energy converter 130 b. Inother embodiments, the second energy converter 130 b can provide otherfractions of the overall electrical power required by the aircraft.

Another expected benefit of the foregoing arrangement is that, byproviding power extracted from both the first shaft 115 a and the secondshaft 115 b to a common bus 140, the operator has a greater degree ofcontrol over which of the aircraft devices 144 a, 144 b can be operatedat any point in time. In particular, with a common bus 140, any of theaircraft devices 144 a, 144 b, and any combination of aircraft devices144 a, 144 b can be operated so long as the combined electrical powerprovided by the first energy converter 130 a and second energy converter130 b is sufficient, without regard as to whether the power is providedby the first shaft 115 a or the second shaft 115 b. This is unlike someexisting arrangements in which the power provided by the two shaftscannot be “mixed” on a common bus (due to frequency/phaseincompatibility), and as a result, each shaft in these existingarrangements can provide power to only a certain subset of aircraftdevices 144 a, 144 b.

A further expected benefit of at least some of the foregoing embodimentsis that the operator can taxi the aircraft on a single engine withouthaving to cut power to the aircraft devices 144 a, 144 b. In particular,the ability to extract some of the required power from the second shaft115 b and provide the power together with power extracted from the firstshaft 115 a on a common bus 140 allows the operator to provide power toany of the aircraft devices 144 a, 144 b using a single engine duringtaxi, which can improve overall fuel consumption. This benefit can alsoextend to in-flight engine-out operation, allowing the operator greaterflexibility in selecting which electrically powered devices receivepower during an in-flight engine shut down. Still further, duringin-flight idle (e.g., during decent), the fan 117 can windmill,providing power to any desired aircraft devices even at idle powersettings. Accordingly, in any of the foregoing embodiments, and inparticular, during ground taxi and idle decent, the operator cancontinue to operate the electrically powered components of the aircraftwithout an uncommanded reduction in electrical loads. For example, theoperator can power a set of electrical devices during cruise or powereddecent, then shift to idle decent without having electrical devicesautomatically shut down due to lack of available power. In anotherexample, the operator can operate the aircraft over a series of flightsegments that include pre-take-off ground maneuvers, take-off, climb,cruise, decent, landing, and post-landing ground maneuvers while the ACpower provided to the electrical components of the aircraft remains at agenerally constant frequency value, and, in a further particular aspectof this example, without any uncommanded reductions in electrical loadcaused by lack of available electrical power.

As is also shown in FIG. 2, the system 110 can include a start converter135 for the first energy converter 130 a, and a generator control unit134 for each of the starter generator 132 and the generator 133. Thestart converter 135 and the generator control units 134 can each beunder the control of an overall controller 136, and any of the foregoingcontrol devices can be computer-based and programmed with instructionsfor carrying out the functions described below. The start converter 135can control the synchronous starter/generator 132 and the first constantspeed drive 150 a. For example, in one embodiment, the start converter135 can be used to drive the starter/generator 132 as a starter ratherthan a generator during an engine start procedure. In addition, thestart converter 135 can control the first constant speed drive 150 a tooperate in a variable speed manner. In particular, the first constantspeed drive 150 a can receive an input from the starter/generator 132and can provide a variable speed output that drives the first shaft 115a. This ability can be particularly useful during cold engine startswhen the oil in the engine is highly viscous. During an initial portionof the start-up procedure, the gear or power ratio of the first constantspeed drive 150 a can be selected to provide high torque at relativelylow RPM to the first shaft 115 a to overcome initially high viscous dragcreated by the cold engine oil and/or other temperature-sensitiveelements of the engine. As the speed of the first shaft 115 a increasesand the viscosity of the oil decreases, the first constant speed drive150 a can be controlled to produce less torque at higher RPM over thecourse of the engine start process. In another embodiment, thecontroller can connect the starter/generator 132 directly to the shaft115 a, bypassing the constant speed drive 150 a entirely (e.g., via aclutch arrangement or other selectable coupling). Then during enginestart, the starter/generator, while being controlled by the startconverter 135, applies a suitable starting torque to the engine to startit in a desirable manner.

Another advantage of the foregoing start capability is that the engine111 can be started by electrical power only. For example, thestarter/generator 132 can be powered by an auxiliary power unit (APU), aground cart, or another device. In any of these embodiments, the engine111 can be started without the need for a separate, pneumatically drivenstarter.

Another potential benefit of the foregoing arrangement is that thestarter/generator 132 and the generator 133 are not mechanicallyconnected directly to each other. Instead, their outputs are connectedvia the bus 140. As a result, the need for a mechanical coupling betweenthe starter/generator 132 and the generator 133 is eliminated. This canavoid potential problems associated with having two generators coupledto a single gearbox that can potentially cause oscillations or otheradverse interactions between the generators, which can damage or reducethe efficiency of the generators. This potential drawback can beeliminated via the foregoing arrangement.

The generator control units 134 can control the starter/generator 132and the generator 133. For example, the generator control units 134 cancoordinate the output of the starter/generator 132 and the generator 133depending upon load requirements, and/or can make adjustments to thepower output provided by the starter/generator 132 and the generator 133in accordance with aircraft power requirements. In some cases, theoutput of the constant speed drives 150 a, 150 b can be adjusted toprovide the same output frequencies. Accordingly, the overall controller136 can control the operation of the constant speed drives 150 a, 150 b.In other embodiments, the systems or subsystems that provide theforegoing control functions can be different, and/or the controlresponsibilities can be shifted from one controller to another, but ingeneral, the overall system can control the frequencies and phaserelationships of the power produced by the converters 130 a, 130 b to bewithin selected ranges and/or limits.

The constant speed drives illustrated in FIG. 2 can take any of a numberof suitable forms. FIGS. 3A and 3B illustrate one arrangement for aconstant speed drive 150 a that includes an input shaft 151 and anoutput shaft 152, each coupled to a variable diameter pulley 157. A belt158 extends around the pulleys 157. As the relative diameters of thepulleys 157 are changed (e.g., by moving mating halves of each pulley157 toward and away from each other, as shown in FIGS. 3A and 3B), thespeed of the output shaft 152 can be maintained at a constant rate,despite a variation in the speed of the input shaft 151.

FIG. 3C illustrates another arrangement for a constant speed drive 150 ain which an input shaft 151 drives a pump 159 and an output shaft 152 isdriven by a motor 160. A fluid path 161 connects the pump 159 and themotor 160. By varying the output of the pump 159 (e.g., the pumpdisplacement), this hydrostatic continuously variable transmission canmaintain a constant rate for the output shaft 152 despite a varyingrotation rate for the input shaft 151.

FIG. 3D illustrates a constant speed drive 150 a configured inaccordance with still another embodiment. In this embodiment, theconstant speed drive 150 a includes a toroidal continuously variabletransmission (CVT). The device is generally similar to CVTs used in theautomotive industry, including but not limited to devices produced bythe Nissan Motor Company, Ltd. of Tokyo, Japan (e.g., the CVT providedwith the 350 GT-8 automobile). Accordingly, the constant speed drive 150a can include an input shaft 151 that rotates in a first direction R1,and an output shaft 152 that rotates in an opposite direction R2. Theinput shaft 151 drives an input disk 153 and the output shaft 152 isdriven by an output disk 154. The input disk 153 and output disk 154have toroidal surfaces that mate with corresponding rollers 155, shownas a first roller 155 a and a second roller 155 b. Each of the rollers155 a, 155 b rotates about an axis A as indicated by arrows R3 and R4,respectively. Each of the rollers 155 a, 155 b is also pivotable about acorresponding pivot axle 156 a, 156 b as indicated by arrows R5 and R6.As the rollers 155 a, 155 b pivot in a mirrored fashion about therespective pivot axes 156 a, 156 b, they change the drive ratio betweenthe input shaft 151 and the output shaft 152. This technique can be usedto create a constant rotation speed for the output shaft 152 while therotation speed of the input shaft 151 varies, so as to provide constantfrequency AC power as described above with reference to FIG. 2. Theforegoing technique can also be used in reverse to vary the speed of theinput shaft 151 given a constant input speed at the output shaft 152,for example, during engine starting, as was also described above withreference to FIG. 2.

The arrangement of the toroidal disks 153, 154 and corresponding rollers155 a, 155 b can produce a constant output speed at the output shaft152, even when the input speed at the input shaft 151 varies by a factorof about five, as is expected to be the case for a typical low pressureaircraft turbine engine shaft. Accordingly, this arrangement is expectedto be suitable for installation in the system 110 described above withreference to FIG. 2.

In still further embodiments, the system 110 can include an energyconverter that electrically converts a variable frequency AC power to aconstant frequency AC power. For example, FIG. 4 illustrates portions ofanother embodiment of the system 110 shown in FIG. 2 that eliminates theconstant speed drives 150 a, 150 b. Instead, the system 110 includes anenergy converter 430 a can in turn include a variable frequencystarter/generator 432. The variable frequency starter/generator 432receives power from the first shaft 115 a during normal operations andprovides power to the first shaft 115 a during starting operations. Thestarter/generator 432 is coupled to a converter 437 that converts powerfrom a variable frequency AC form to a constant frequency AC form.During the engine start mode, the converter 437 can act as the startconverter, driving the starter/generator 432 to apply a suitablestarting torque to the engine to start it in a desirable manner. Forexample, the converter can include a variable speed, constant frequencyinverter such as are available from several suppliers, includingHamilton Sundstrand of Windsor Locks, Conn. Other aspects of the system110, including arrangements for extracting power from the second shaft115 b, can be generally similar to the arrangements described above withreference to FIG. 2. The particular arrangement of the energy converter(e.g., whether it includes a mechanically-based constant speed drive, aninverter, or another device) can be selected based on criteria thatinclude but are not limited to system weight, system coolingrequirements, and overall system efficiencies.

From the foregoing, it will be appreciated that specific embodiments ofthe disclosure have been described herein for purposes of illustration,but that various modifications may be made without deviating from thedisclosure. For example, the constant speed devices can haveconfigurations other than those specifically shown and described above.The engines, generators, starter/generators, controllers, and/or othersystem components can be applied to aircraft having configurations otherthan those described above. In still further embodiments, the foregoingsystems and components can be applied to non-aircraft power generationarrangements.

Certain aspects of the disclosure described in the context of particularembodiments may be combined or eliminated in other embodiments. Forexample, while aspects of the foregoing systems were described in thecontext of two-spool engines (e.g. engines having a high pressure spooland a low pressure spool), in other embodiments, similar principles canbe applied to three-spool engines or engines having otherconfigurations. In context of a three-spool engine, a separate energyconverter can be coupled to each of the high pressure and low pressurespools and optionally, to an intermediate pressure spool as well.Further, while advantages associated with certain embodiments have beendescribed in the context of those embodiments, other embodiments mayalso exhibit such advantages, and not all embodiments need necessarilyexhibit such advantages. Accordingly the disclosure can include otherembodiments not explicitly shown or described above.

1. An aircraft system, comprising: an engine that includes: a firstshaft; and a second shaft; a starter/generator; a first constant speeddrive connected between the first shaft and the starter/generator, toreceive a first mechanical input rotating at a first variable rate, andprovide a first mechanical output rotating at a first generally constantrate; a generator; a second constant speed drive connected between thesecond shaft and the generator to receive a second mechanical inputrotating at a second variable rate, and provide a second mechanicaloutput rotating at a second generally constant rate; and a bus systemelectrically connected between the starter/generator and the generator,to receive electrical power from the starter/generator and thegenerator.
 2. The system of claim 1 wherein the second constant speeddrive includes a mechanical, continuously variable transmissionconnected between the first shaft and the starter/generator.
 3. Thesystem of claim 2 wherein the mechanical, continuously variabletransmission includes an input shaft, an output shaft, and a pivotableroller operatively coupled between the input shaft and the output shaft,and wherein the roller is pivotable between a first position with afirst drive ratio between the input shaft and the output shaft, and asecond position with a second drive ratio between the input shaft andthe output shaft, and wherein the first and second drive ratios differby a factor of about five or greater.
 4. The system of claim 1 whereinthe first constant speed drive includes a first mechanical, continuouslyvariable transmission connected between the first shaft and thestarter/generator, and wherein the second constant speed drive includesa second mechanical, continuously variable transmission connectedbetween the second shaft and the generator.
 5. The system of claim 4wherein the first mechanical, continuously variable transmissionproduces a constant output speed given an input speed that varies by upto a factor of about two, and wherein the second mechanical,continuously variable transmission produces a constant output speedgiven an input speed that varies by a factor of up to about five.
 6. Thesystem of claim 1 wherein the second generally constant rate isgenerally the same as the first constant rate.
 7. The system of claim 1wherein the starter and the starter/generator provides generally thesame frequency of electrical power.
 8. The system of claim 1 wherein thebus system includes a first bus portion coupled to thestarter/generator, a second bus portion coupled to the generator, and aswitch between the first and second bus portions.
 9. The system of claim1 wherein the second shaft is positioned annularly around the firstshaft.
 10. An aircraft system, comprising: an engine that includes: afirst shaft; and a second shaft; a bus system; a first energy convertercoupled between the first shaft and the bus system, including astarter/generator to convert a first variable frequency energytransmitted by the first shaft to a first generally constant frequencyenergy; and a second energy converter coupled between the second shaftand the bus system, the second energy converter including a generator toconvert a second variable frequency energy transmitted by the secondshaft to a second generally constant frequency energy.
 11. The system ofclaim 10 wherein the first energy converter includes a mechanical,continuously variable transmission connected between the first shaft andthe starter/generator, and wherein the transmission includes a variablerotation rate input shaft and a constant rotation rate output shaft. 12.The system of claim 10 wherein the starter/generator includes a variablefrequency generator, and wherein the first energy converter includes anelectrical inverter coupled to the starter/generator to receive variablefrequency electrical power and produce a constant frequency power. 13.The system of claim 10 wherein the second energy converter includes amechanical, continuously variable transmission connected between thesecond shaft and the generator, and wherein the transmission includes avariable rotation rate input shaft and a constant rotation rate outputshaft.
 14. The system of claim 10 wherein the first energy converterincludes a first mechanical, continuously variable transmission coupledbetween the first shaft and the starter/generator, and wherein thesecond energy converter includes a second mechanical, continuouslyvariable transmission coupled between the second shaft and thegenerator.
 15. The system of claim 14 wherein the first mechanical,continuously variable transmission produces a constant output speedgiven an input speed that varies by up to a factor of about two, andwherein the second mechanical, continuously variable transmissionproduces a constant output speed given an input speed that varies by afactor of up to about five.
 16. The system of claim 15 wherein the firstgenerally constant frequency energy and the second generally constantfrequency are generally at the same frequency.
 17. A method foroperating an aircraft system, comprising: starting an engine by drivinga first shaft with a starter/generator; extracting a first portion ofthe energy from the first shaft with the starter/generator; extracting asecond portion of energy from a second shaft of the engine; convertingthe first portion of energy from a first variable frequency to a firstfixed frequency; converting the second portion of energy from a secondvariable frequency to a second fixed frequency; and distributing thefirst and second portions of energy to aircraft components via a bussystem.
 18. The method of claim 17 wherein the second fixed frequency isgenerally identical to the first fixed frequency.
 19. The method ofclaim 17, further comprising extracting the first portion of energyusing a mechanical, continuously variable transmission.
 20. The methodof claim 19, further comprising starting the engine by varying a speedratio between an input of the continuously variable transmission and anoutput of the continuously variable transmission as the engine starts.21. The method of claim 17 wherein the aircraft system has more than oneengine, and wherein the method further comprises: operating only asingle engine of the aircraft during taxi maneuvers; and operatingelectrically driven equipment on the aircraft during the taxi maneuverswithout reductions in electrical load that are not commanded by theaircraft crew.
 22. The method of claim 17, further comprising: operatingthe engine during a cruise phase; operating the engine during an idledecent phase; and operating electrically driven equipment on theaircraft during the cruise and idle decent phases without reductions inelectrical load that are not commanded by the aircraft crew.
 23. Themethod of claim 18, further comprising maintaining the fixed frequencyat a generally constant value over a flight that includes pre-takeoffground maneuvers, takeoff, climb, cruise, decent, landing andpost-landing ground maneuvers.
 24. The method of claim 17, furthercomprising operating the engine at an idle speed that is independent ofa load placed on the engine by distributing the first and secondportions of energy to aircraft components.
 25. The method of claim 17,further comprising: operating a plurality of electrically driven devicesduring decent and landing; and taxiing the aircraft after landingwithout reducing the number of operating electrically driven devices inelectrical load in a manner that is not commanded by the aircraft crew.